J. Smooth Muscle Res. (2012) 48 (4): 65–95 65 Review Cells of visceral smooth muscles Giorgio GABELLA1 1University College London, U.K. Received July 9, 2012; Accepted July 23, 2012 1. Introduction 2. Different types of muscle cells 3. Visceral muscle cells 4. Musculature of ileum 5. Ileal circular muscle as a tissue 6. Quantitative data on muscle structure 7. Innervation of ileal musculature 8. Variation in muscle innervation 9. Detrusor muscle of bladder 10. Innervation of rat bladder detrusor 11. Glial cells in detrusor muscle 12. Non-muscle cells in detrusor muscle 13. Nerve endings in ileal musculature 14. Non-muscle cells in ileal musculature 15. Non-muscle cells in ileal musculature 16. Other visceral muscles 17. Conclusions References 1. Introduction There is an astonishing variety of smooth muscles in the wall of viscera and vessels and in other organs; a list touches on every part of the body, with the exception of the central nervous system (Table 1). As to the size of smooth muscles, the upper limit is related to the body size of the animal, and so the range in the volume, say, of the myometrium or the media of the aorta, from a shrew to a whale, is im- mense, even when the muscle serves a similar function. The lower limit in size is a single muscle cell, Correspondence to: Dr Giorgio Gabella, MD DSc, Department of Cell and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK e-mail: [email protected] An essay on the fine structure of the visceral musculature and the cells found in it. The paper is a personal ac- count based mainly on the research work of the author, as published, and was presented, in a modified form, at a meeting organized by Prof. Hikaru Suzuki and Prof. Terumasa Komuro in Miyazaki, Japan, February 2010. ©2012 The Japan Society of Smooth Muscle Research 66 G. GABELLA in effect an independent, self-contained contractile or motor unit. There are important variations in his- tologic architecture, many of them imposed by size constraints and functional roles (for example, the structural asymmetry between intimal and adventitial side of every muscle cell in small tubular organs, such as arterioles) (Bülbring et al., 1981). Yet, all smooth muscle cells are relatively similar —in size, chemical composition, electrical properties and mechanical properties, in so many animal species, organs and functions. The total amount of smooth muscle in the human body is well over a kilogram (Table 2). There could be something in the region of 200-300 billion muscle cells in the human body. A standing comparison with skeletal muscle has dominated the investigation of smooth muscle (striated muscles being by far better known and easier to study), because both of them, together with cardiac muscle, are powered by a protein motor made of actin and myosin. In general terms, striated muscle is a good foil to highlight the properties of smooth muscles. In skeletal muscles, the fibres are slaves to the nerve that reaches them at the motor end plates. What the nerve dictates, the skeletal muscle fibre does. And —in an extreme simplification— either the nerve is conveying impulses and the skeletal muscle fibre then contracts, or the nerve is silent and the muscle fi- bre is then relaxed. This resembles a digital system, with only two possible states. In contrast, a smooth muscle cell receives a variety of stimuli, integrates them, and responds with a contraction whose na- ture depends on the results of calculations. Unlike skeletal muscle, smooth muscle resembles more an analogue system. Smooth muscle is sensitive to circulating substances (hormones), to substances released locally (local hormones), to transmitters from adjacent nerves and other cells, to temperature, to stretch; in some smooth muscles —but surely not in all— there is ionic coupling between muscle cells; and many muscles have some myogenic activity. Muscle cells are relatively independent movers that receive and act upon a variety of stimuli. In addition, the mechanics of skeletal muscles (long cylindrical fibres, tendons, bone insertions, fasciae) appears simple by comparison with that of smooth muscles (minute spindle-shaped cells, no proper tendons, diffuse innervation, no fascia or capsule, many intercellular junctions). Both striated muscle fibres and smooth muscle cells are the effectors of muscular organs, the active players in spe- cialized mechanical events; however, the mechanical properties of cells and those of fibres are funda- mentally different. Furthermore, smooth muscles, made though they are mainly (and sometimes entirely) of muscle cells, can comprise several other cell types, as I will discuss. That this should not be the case with skeletal (and cardiac) muscles is dubious; nevertheless, the non-muscle cells of smooth muscles have attracted a particular interest and much experimental work. The present account of the structure of visceral muscles is focused at the cellular level, which may be an optimal one for understanding their physiology. Smooth muscles are tissues (the subject of histol- ogy); they are of mesenchymal origin (except for smooth muscles in the head and neck regions, which are of neural crest origin), and are made of muscle cells and other cell types. A smooth muscle, there- fore, can be imagined as a society in equilibrium, with competition and co-operation between its cells, and much interaction. The functional performance is the outcome of a certain population dynamics, rather than the expression of the activity of any individual component. The relative position of various cells and their relations are crucial to the specific muscle function. The production of mechanical work by the protein motors of muscle cells and the interaction between the cells of the muscle are the two key aspects of these muscles’ physiology. Visceral smooth muscles 67 Lastly, it is easy to underestimate the extent of the dynamic qualities of smooth muscles. Not only there are active and forceful changes in shape during muscle contraction (as well as passive changes in shape); there is also developmental growth and differentiation, which extend well into adulthood; and, throughout life, there is adaptive growth (and atrophy), which adjusts the muscle structure in response to actual functional demands. While the individual history leaves a mark in the structure of every single muscle, the evolutionary history is embedded in the potential for growth and adaptation present in these muscles. 2. Different types of muscle cells As to the muscle cells themselves, the question arises whether, in a given muscle, they are all equivalent – functionally, metabolically, biochemically and in origin and fate. Early suggestions, for example of light and dark muscle cells, were inconsequential. In fact, the evidence available indicates that the muscle cell population is uniform within a given muscle (or muscle layer), without suggestions of clearly different muscle cells types (sub-populations of cells). Nevertheless, this uniformity is ill de- fined, and there are questions that remain to be explored. How uniform is the activation of muscle cells, within a defined population? How synchronously do they respond to stimulation? How are contraction and relaxation cycles matched in adjacent cells? Muscles with high electrical (ionic) coupling, such as the ileal circular musculature, are said to behave like a syncytium, which is an oversimplification but also a pointer to a uniformity in their response to stimuli. Smooth muscle cells are shown in every il- lustrations in this essay, and the images bear out the substantial uniformity of appearance. Concerning the embryonic development of this mesenchymal tissue, the steps of differentiation are not yet well outlined and the separation of muscle cells from other cell lineages, especially those of other cell types present in the muscle itself, is poorly understood, but this is being investigated (Ger- shon, 2010). As to the late stages of differentiation of visceral muscles, for example, day 8 chick embryo giz- zard or day 18 rat embryo ileum, all the major cell types can be recognized at that time; at the ultra- structural level the muscle cells appear to be all at an identical stage of differentiation, namely, they seem to be maturing simultaneously and at the same rate. Precursors or undifferentiated muscle cells are not left behind within the muscle for later development (unlike in some striated muscles). Although the longitudinal musculature develops later than the circular musculature in the intestine, the two muscles subsequently differentiate and mature at the same rate. The uniformity of the muscle cell population is compatible with the occurrence of mitosis in ma- ture muscle cells and with the degeneration of some muscle cells: both cell division and cell death are occasionally observed in any visceral muscle at all times of life. Also, in overload hypertrophy, visceral muscle cells not only can divide but also can double or treble their volume; however, some cells grow more than others and cell sizes come to spread over a wider range than in control tissues. 68 G. GABELLA Table 1. Distribution of smooth muscles Table 2. Rough estimates of the amount (in grams) of smooth musculature in the human body, giving a total of 1,000–1,600 grams or 1.3– 2.2% of the body weight bladder 30–60 g ureter, vesicles, vas 30–60 g uterus 200–400 g gut 700–1,000 g airways 50–100 g vessels 150–300 g skin 10–30 g diffuse musculature 50–100 g 3. Visceral muscle cells In situ and when not contracted, visceral muscle cells are very slender, more than a hundred times as long as they are wide, at considerable variance from the shape of many of these cells when they are excised and isolated in vitro, as single cells or as small bundles.